Mastodawn

Physical security and cryptography can learn from each other, part 11367:

Hotels wisely don't put the room number on guest keycards so if someone finds your card, they'd have to exhaustively search the hotel to find the room it opens.

Some hotels now have elevators programmed to only let you call the floor for which your keycard is coded, preventing guests from wandering to other floors.

But it also means the elevator can be used as an efficient oracle to determine the floor of a found key.

Matt Blaze 18h ago

In other words, restricting the elevator in this way is a bad tradeoff. It makes it harder for guests to visit their friends on other floors, but it reduces the complexity for an outsider burglar from O(|rooms|) to O(|floors|) + O(|rooms_per_floor|), a much more feasible search space.

Matt Blaze 4h ago

(The point here is not about securing any specific hotel or keycard system. The point is that thinking about security in abstract terms can reveal properties and weaknesses that aren't otherwise obvious.)

rag. Gustavino Bevilacqua 4h ago

In the holiday house we rent there is a masonite panel in a closet under the stairs, with screws in the corners.
Since it seems to hide something (actually it hides just the heating pipes) I wrote on the wall behind it: "The safe is not here! Try again!".

Bernard Sheppard 18h ago

@mattblaze I am in a hotel now (in Japan, for context).

I observed that you could access any floor when my backpack pressed several floor buttons on our first ride.

When I later attempted to access the laundry room floor but could not, but could access my floor, thought that perhaps the first observation was an anomaly associated with the fact that the only other elevator was being attended by an elevator repairman at the time of the multiple floor incident.

It turns out that I had my Suica card in my hand, not my hotel card, had selected my floor based on the swipe of another guest in the elevator, but was unable to select the laundry floor after a time out.

I discovered this when I couldn't open my room with the Suica.

The flaw in this hotel is that one swipe enables multiple floors, defeating the security access aspect while providing the anonymity. A guest can swipe, and an intruder can then access a floor that they have previously observed a target accessing, and then, presumably, having determined the room number via other (social engineering) means, door knock with "hotel engineering".

Ysegrim 12h ago

@BernardSheppard @mattblaze In a hotel I stayed in a few years back, someone discovered an interesting hack: while you could only select a floor after swiping your card (IIRC and only your own), after someone had selected a floor you could select any additional floor by pushing the button of the already selected floor and the new floor at the same time, thanks to the physical wiring of the card-reader add-on.

Not sure whether you'd count that wiring as "software bug" or "physical security issue" :)

mfdeakin 16h ago

@mattblaze I enjoy the idea, but are you sure they don't print the room number for security reasons? I was under the impression it was because they reprogrammed them when they gave them to you

Carolyn 15h ago

@mfdeakin @mattblaze Both could be true?

🔶Mark Nicoll 3.5%🏴󠁧󠁢󠁥󠁮󠁧󠁿🇬🇧🇪🇺🇺🇳10h ago

@mfdeakin @mattblaze
they do program them before they hand them to you, but the reason for that is security. They could just program a specific key for every room and put the room numbers on them, but that is considered bad practice.

Mark Otway 6h ago

@duckwhistle
Given how often people lose or forget to return their key cards when they check out, having room-specific keys isn't really a logistically sound idea.
@mfdeakin @mattblaze

Alan Bellingham 7h ago

@mfdeakin @mattblaze It's easy enough to decide. Are hotels interested in security or in cost? If the room number was on the key, it's extra cost to manufacture, it's extra cost because you'd need twice as many to allow for losses, it's extra cost because you'd need racks to store them, it's extra cost because reception would have to sort returned keys

So instead of having the room number on the key, it's quickly handwritten on the card folder, and you'd never lose that with the key

WellsiteGeo 6h ago

@mfdeakin @mattblaze
They don't print the room number because they don't have a printer for the cards. (And there's probably a policy against marker pens on stationary orders, for this reason.)

ᴮᵉⁿ ᴿᵒʸᶜᵉVOTE IN THE PRIMARIES 15h ago

the solution is for the hotel itself to drop keycards around the hotel and in the surrounding area

then when that honeypot keycard is used on the elevator it takes the potential burglar to the basement where a burly guy named Steve is waiting for them with a grin

Pete / Syllopsium 7h ago

@benroyce @mattblaze It's a funny idea, but as I'm sure you'll know this isn't like dropping your file/USB stick, it's not unlikely a customer will misplace their card and then 'miraculously find it again' in the surrounding area, as that's what they walked past

ᴮᵉⁿ ᴿᵒʸᶜᵉVOTE IN THE PRIMARIES 5h ago

@syllopsium @mattblaze

well if i was being serious, the burly guy in the basement would have been named Bob

Pete / Syllopsium 4h ago

@benroyce @mattblaze Well, yeah, Steve is just too nice, isn't he?

Trammell Hudson 9h ago

@mattblaze I've also seen some hotel elevators where you swipe your keycard and it selects the correct floor for you, removing the O(floors) component.

Ariadne Conill 🐰

@th @mattblaze yeah i encountered that recently in germany and was just like ????????????? why

Enola Knezevic 9h ago

@ariadne @th @mattblaze What if you wanted to have a drink at the rooftop bar before going to your room?

Ariadne Conill 🐰

@rhelune @th @mattblaze exactly

@ariadne @rhelune @th @mattblaze floors with shared amenities are usually open to all cards (provided you have one). Some hotels also doing restric your floors, but simply required a valid card to take the elevator, trying to prevent non-customer coming into the hotel from being able to wander in the corridors. This is obviously defeated by stairs (that are usually present) or simply by riding the elevator with other people.

Enola Knezevic 5h ago

@halfa @ariadne @th @mattblaze Sure but, if you scan keycard because there is a scanner in the lift and press the top floor to go to the bar, you do not want to be automatically taken to the third floor.

@rhelune
Oh, no hotel will restrict access to a bar. They're always free floors.
@ariadne @th @mattblaze

Enola Knezevic 9h ago

@hypostase @ariadne @th @mattblaze Yes but you do not want to be taken to the wrong floor just because you swiped your keycard. If the lift acted that way, I would suspect a prank (or worse): https://youtu.be/1Un_oHaf798

Ylvis | The Intelevator - Episode 1 (Exhibition) | discovery+ Norge

YouTube

@rhelune
I was almost expecting the Scotsmen.
@ariadne @th @mattblaze

Velocipede Rider 7h ago

@rhelune Also annoying if you are staying at a hotel with a group of friends (e.g. for an event). Then you cannot easily go to their floor and have to always meet in the lobby.

EDIT: OK I am an idiot, @mattblaze already covered this exact point! 🤪

>… harder for guests to visit their friends on other floors…

WellsiteGeo 6h ago

@ruari @rhelune @ariadne @th @mattblaze
Hotels don't want guests visiting each other's rooms. They want guests meeting each other profitably in the bar. (Also make it easier to charge the prostitutes their ground rent.)

Mark Otway 6h ago

@mattblaze
Given the recent news about one hotel chain giving a rando a keycard just by knowing the occupant's name, I'm not sure any hotel security should be trusted.

Matthew van Eerde 18h ago

@mattblaze I suspect there is a square-root law here, where optimum balance between the "wandering guest" threat and the "found keycard" threat is achieved by allowing elevator access to the square root of the total number of floors (your own, plus some randomly selected floors)

robloblaw 10h ago

@mvaneerde @mattblaze
The maximal security approach is for the key card to only given access to a random floor (excluding the floor the room is on).

Enola Knezevic 9h ago

@mvaneerde @mattblaze not counting the reception floor, the wellness floor, the restaurant floor, and the garage floor, of course

Rihards Olups 6h ago

@mvaneerde @mattblaze Is this the overengineering we sometimes hear about ? :)

On a more serious note, that would probably be immediately offset by half the staff being confused, and many guests both getting lost, and complaining to the front desk.
Guests have learned this feature. Some (many?) even rely on it to avoid remembering the floor - scan the card, smash some buttons, done.
Now they would end up on semi-random floors...

Matthew van Eerde 4h ago

@richlv @mattblaze all valid concerns. In the "pro" column I will add guests would have more recourses if the ice machine on their floor breaks

Rihards Olups 2h ago

@mvaneerde @mattblaze Do the random floors change every time guests use the lift? :)

In a way that "feature" makes me feel less safe. What do you mean, in your hotel it is dangerous and there are random burglaries or something?

Sidenote: "ice machine" seems like such an American thing. I recall my first visits to the USA many years ago, and

Can Acar 18h ago

@mattblaze unfortunately, there are people who would interpret this to mean "it is now OK to print room numbers on keys".

Jeff Grigg 18h ago

@canacar @mattblaze

Yea; why not?
So many of the guests keep their key cards in the paper wrapper -- which has their room number on it. 🙄

Hoarse e-Books 17h ago

@JeffGrigg @canacar @mattblaze Came here to say exactly this.

@JeffGrigg @canacar @mattblaze

But how else will I remember what room I am in at midnight after a few drinks?

Everything looks the same.

Jeff Grigg 16h ago

@print @canacar @mattblaze

Practical advice: Put your hotel room key in a different pocket than the holder. (The paper holder has your room number on it.)

Enola Knezevic 9h ago

@JeffGrigg @print @canacar @mattblaze Take a photo of the paper sleeve, leave it in the room. I always know which room is mine by the "do not disturb" hanger, additionally, the thief is less likely to try such a room.

Norman Wilson 3h ago

@rhelune @JeffGrigg @print @canacar @mattblaze Take a photo of the Do Not Disturb card too, just in case.

Bill Zaumen 14h ago

@print @JeffGrigg @canacar @mattblaze I've seen worse than not remembering which room you were in. On a trip to southern France, I had to get up early on my final day to get to the airport. I took a cab. As I was about to get in some shirtless British guy, obviously up all night, was asking for help - he couldn't remember where his hotel was (and probably not its name either), and was quite rude when the driver couldn't help. I mentioned a map at the train station (if only to get rid of him)

Once In winter, we went wine tasting staying in cabins.

Followed by dinner and had a beer tasting paddle. I don't normally drink much.

The boys went for a walk, in the dark found a oval, then the pool. Of course we jumped in.

Cold and Dripping wet, ran back to the cabins. In the door, into the shower to turn it on to warm up. Went to go through the sliding door to the bedroom. It was stuck. Tried to get arm in to get it unstuck. Looked behind me to the kitchen table.
(Cont)

Hang on.

Thats not our stuff!

Clothing and alcohol bottles covered the table.

Through the door crack I could see there are two unknown girls in the double bed.

"Opps, Sorry, wrong cabin"

Turned shower back off and ran out still dripping.

Found right cabin open was. Had shower and got dressed. But then couldn't find the keys.

Retraced steps. Found them in the middle of the oval 45 minutes later.

Didn't tell the wife or the rest of the group we were with until a decade later.

@canacar Friends were drunk and we saw them get into a taxi on the other side of the street from the hotel. Five minutes later, they were dropped off at the hotel by the taxi driver, who’d taken them around a few blocks.

@mattblaze same applies to i.e. self-storage units.

Norman Wilson 3h ago

@kkarhan @mattblaze Isn't a hotel room a self-storage unit?

Hoarse e-Books 17h ago

@mattblaze A week ago I checked into a hotel in Vietnam that put little stickers with the room number on our keys. Removing these was Job One once we got into the room.

Richard W. Woodley ELBOWS UP 🇨🇦🌹🚴‍♂️📷 🗺️17h ago

@mattblaze
They did that for office buildings too. You had to scan your card in he elevator and could only choose the floors you weer allowed on. Solution - ride the elevator till someone gets off on the floor you want.